Citation

Gong Jiang, Zheng Fengling, Zhang Hanqing, Shu Weihan, Wang Hao, Zhang Ni, Huang Pengbing, Zhang Chuancai, Dai Bin. The interfacial effect of SiO₂-Ni₃Mo₃N efficiently catalyzes the low-temperature hydrogenation of dimethyl oxalate to ethanol[J]. Chin. Chem. Lett., 2025, 36(8): 111122  

Jiang Gong , Fengling Zheng , Hanqing Zhang , Weihan Shu , Hao Wang , Ni Zhang , Pengbing Huang , Chuancai Zhang ^,*, Bin Dai ^,*     

The conversion of syngas to recycle factory waste gases and enable the clean utilization of coal offers a viable solution that aligns with green development goals and promotes a sustainable energy economy [1,2]. Ethanol (EtOH), produced through the conversion of syngas to dimethyl oxalate (DMO) followed by hydrogenation, finds significant applications in the healthcare and food industries and is also considered a potential alternative for automotive fuel [3–5]. In recent decades, considering the realization of industrialization of synthesis gas DMO and the wide application of EtOH, substantial research has been carried out on DMO hydrogenation to EtOH [6–8]. However, achieving high EtOH yields at low temperatures remains a major challenge. Current yields of DMO hydrogenation to EtOH at low temperatures are still inadequate [8,9], hindering the progress toward industrial-scale EtOH production from syngas. Therefore, the research and development of DMO highly efficient catalyst for EtOH production at low temperature has become an urgent need for industrial realization of EtOH production from syngas.

The further hydrogenation products of DMO primarily include methyl glycolate (MG) [10,11], methyl acetate (MA) [12], ethylene glycol (EG) [13,14], and EtOH. Effective deep hydrogenation to produce EtOH requires a catalyst with robust capabilities for H₂ adsorption and dissociation, as well as the adsorption and activation of C-O and C=O bonds. Such a catalyst must feature a multifunctional interface to facilitate these processes. Copper (Cu)-based [15,16] catalysts are favored for their excellent Cu⁰ dissociation ability towards H₂ and Cu⁺ adsorption and activation ability towards C=O. Despite their effectiveness, Cu-based catalysts often require high reaction temperatures, leading to sintering issues, resulting in poor catalytic stability which hampers practical applications [17,18]. In contrast, non-Cu-based catalysts such as iron (Fe) [6,19], nickel (Ni) [5,20], and molybdenum (Mo) [8,9], offer better EtOH yields and improved stability but necessitate high temperatures (230–300 ℃) and pressures (up to 2–3 MPa). Therefore, achieving high EtOH yields typically involves challenging conditions of elevated temperature and pressure, presenting significant challenges for developing an effective catalytic interface.

Our previous studies have demonstrated the exceptional performance of the Ni₃Mo₃N catalyst in the hydrogenation of DMO to EtOH. Nonetheless, significant quantities of the intermediate product MA are produced at low temperatures, underscoring the need for improvements in low-temperature hydrogenation performance. Thus, designing a catalyst that enhances the adsorption and activation of intermediate products such as MG and MA is essential for achieving high EtOH yields at reduced temperatures. Research has demonstrated that metal-oxide interfaces are pivotal in various catalytic reactions and can substantially improve catalyst activity [21,22]. SiO₂, a widely used oxide support, is effective in numerous challenging reactions [23,24]. Additionally, SiO₂ provides appropriate acidic sites [25,26] that facilitate the adsorption and activation of C=O and C-O species, thereby enhancing EtOH yields.

In this study, a catalyst with rich SiO₂-Ni₃Mo₃N catalytic interface was synthesized for DMO hydrogenation of EtOH. The catalyst could achieve 97.5% EtOH yield at 210 ℃. Studies have shown that SiO₂ is uniformly dispersed on the surface of Ni₃Mo₃N, mainly in the form of Si-O-M (metal). This special structure promotes the transfer of electrons, adjusts the metal-band center of the catalyst, and promotes the adsorption of the catalyst to raw materials and intermediate products, thus significantly improving the ability of the catalyst to generate EtOH. At the same time, the introduction of SiO₂ significantly increased the content of Lewis acid sites (L-acid) and Brønsted acid sites (B-acid) sites in the catalyst, which was also conducive to the generation of EtOH. The catalyst achieved high EtOH yield at low temperature, providing new insights for the construction of highly active DMO hydrogenation catalyst, and providing guidance for the industrialization of synthesis gas to EtOH.

The raw materials used in this experiment, the preparation method of the catalyst and the corresponding characterization method are shown in Supporting information.

The catalyst was initially characterized using X-ray diffraction (XRD), as shown in Fig. S1 (Supporting information). The diffraction peak associated with amorphous SiO₂ (PDF #29–0085) was detected for pure SiO₂. Conversely, for Ni₃Mo₃N and SiO₂-Ni₃Mo₃N catalysts, only the diffraction peak of Ni₃Mo₃N (PDF #49–1336) was observed, while the amorphous diffraction peak of SiO₂ was not detected. This suggests that SiO₂ is highly dispersed on the surface of Ni₃Mo₃N and does not alter the catalyst's structure, thereby facilitating the formation of a more efficient SiO₂-Ni₃Mo₃N catalytic interface. To further explore the microstructural impact of SiO₂ introduction, scanning electron microscopy (SEM) analysis was performed. Fig. S2 (Supporting information) presents SEM images depicting the states of the catalysts before and after SiO₂ incorporation. The results indicate that Ni₃Mo₃N primarily displays a prismatic particle structure, uniformly coated with spherical SiO₂ particles, forming a rich interface between SiO₂ and Ni₃Mo₃N. Additionally, the element mapping distribution (EDX) (Figs. 1a–g and Figs. S3a-h in Supporting information) clearly confirms the successful incorporation and homogeneous dispersion of SiO₂ on the Ni₃Mo₃N surface. Furthermore, the consistent distribution of Ni, Mo, and N elements, along with the presence of lattice fringes corresponding to the 221 crystal faces of Ni₃Mo₃N (Fig. 1h and Fig. S3i in Supporting information), corroborates the formation of Ni₃Mo₃N. The HRTEM image did not reveal lattice fringes associated with SiO₂, indicating amorphous SiO₂ on the Ni₃Mo₃N surface, consistent with the XRD results.

Figure 1

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Fig. 1. Elemental mapping distribution diagrams (a-g) and high-resolution image (h) of SiO2- Ni3Mo3N catalysts.

Simultaneously, XPS analysis was performed to examine the influence of SiO₂ incorporation on the chemical state of elements in the catalyst. The XPS general spectrum indicated the presence of Si 2p orbitals (Fig. S4 in Supporting information), verifying the successful integration of SiO₂. High-resolution photoelectron spectra for Ni 2p, Mo 3d, and Si 2p are illustrated in Figs. S5a-c (Supporting information). Peak fitting, informed by relevant literature, is summarized in Tables S1-S3 (Supporting information). In the Ni 2p (Fig. S5a) photoelectron spectra of the Ni₃Mo₃N catalyst, peaks at 852.3 eV for Ni⁰ and 855.5 eV for Ni²⁺ were identified [27,28]; however, SiO₂ introduction shifted the Ni⁰ peak position leftward by approximately 0.2 eV. A similar pattern was observed in Mo 3d (Fig. S5b) spectra, where the introduction of SiO₂ caused a leftward shift of approximately 0.3 eV for Mo⁰ [29]. This shift may result from electron transfer from Ni₃Mo₃N to SiO₂, leading to increased binding energy for Ni and Mo. Furthermore, the N 1s photoelectron spectra also demonstrated a shift towards higher binding energy (Fig. S5e in Supporting information). Similarly, this trend was observed in the photoelectron spectra of Si 2p orbitals (Fig. S5c). Compared to pure SiO₂, the Si 2p orbits of SiO₂-Ni₃Mo₃N were deconvoluted into two peaks at 103.3 eV for SiO₂ and 101.9 eV for SiO_X (corresponding to Si¹⁺, Si²⁺, and Si³⁺) [30]. The electron transfer between Ni₃Mo₃N and SiO₂ facilitated the formation of an interface between them. Additionally, O 1s photoelectron spectroscopy reveals the presence of a uniform SiO₂ coating on the surface of Ni₃Mo₃N, forming an interface known as SiO₂-Ni₃Mo₃N. The O 1s photoelectron spectrum can be attributed to metal-O bonds, C-O bonds, and C=O bonds [27,28]. Upon the introduction of SiO₂, there is a significant reduction in the peak area corresponding to metal-O bonds (Fig. S5f in Supporting information).

Moreover, H₂-TPR analysis was conducted to further validate the formation of the SiO₂-Ni₃Mo₃N interface. Fig. S6a (Supporting information) shows that the introduction of SiO₂ causes the reduction peak of the catalyst to shift towards higher temperatures. This shift likely results from the enhanced interaction between SiO₂ and Ni₃Mo₃N [31], facilitated by electron transfer, thereby promoting the formation of Si-O-M (metal) bonds. The results indicate that the interface structure between SiO₂ and Ni₃Mo₃N primarily exists in the form of Si-O-M. Additionally, Fourier transform infrared spectroscopy (FTIR) (Fig. S6b in Supporting information) identified vibrational peaks around 1052 cm^-1, corresponding to the presence of Si-O-M groups [32]. Collectively, these results indicate that SiO₂ is effectively dispersed on the surface of Ni₃Mo₃N, forming a well-defined and abundant SiO₂-Ni₃Mo₃N interface.

Fig. 2a illustrates the catalyst performance before and after SiO₂ loading. Under identical reaction conditions, the Ni₃Mo₃N catalyst produces a significant amount of intermediate product MA with a selectivity of approximately 25.1%. However, the incorporation of SiO₂ effectively converts nearly all MA into EtOH, resulting in a remarkable EtOH yield of 97.5%. This observation suggests that the introduction of SiO₂ facilitates the adsorption activation of intermediate product MA and promotes its subsequent hydrogenation to produce EtOH. The performance comparison under optimal conditions also yields identical results (Fig. S7 in Supporting information). In-situ FTIR of MA adsorption characterization was conducted. As shown in Fig. 2b, the stretching vibration peaks at around 1784 and 1275 cm^-1, corresponding to C=O and C-O bonds respectively [33], were observed for gaseous MA. After MA adsorption, noticeable redshifts in the stretching vibration peaks of C=O and C-O bonds were particularly pronounced in the SiO₂-Ni₃Mo₃N catalyst, suggesting that the presence of SiO₂ enhances MA adsorption and facilitates its hydrogenation to produce EtOH [34]. Furthermore, in-situ FTIR characterization of DMO and MG adsorption demonstrated that SiO₂ introduction promotes the adsorption of raw material DMO and intermediate product MG (Fig. S8 in Supporting information), significantly improving EtOH yield.

Figure 2

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Fig. 2. Comparison of performance of different catalysts. (a) Conversion, selectivity, and yield. Reaction conditions: Treaction = 230 ℃, Preaction = 2 MPa, WLHSVDMO = 0.2 h-1, H2/DMO (mol) = 200. In-situ FTIR characterization of MA adsorption (b), NH3-TPD characterization (c), and Py-FTIR (d).

SiO₂ introduction enhances the adsorption of raw material and intermediate products (MG, MA), likely due to increased acidic sites, as confirmed by ammonia temperature-programmed desorption (NH₃-TPD) analysis (Fig. 2c) and subsequent quantitative calculations. The results are presented in Table S4 (Supporting information). These findings demonstrate that SiO₂ incorporation increases the content of acidic sites, particularly weak acidic sites (NH₃ desorption peak at 100–300 ℃) and medium-strong acidic sites (NH₃ desorption peak at 300–600 ℃). Weak acidic sites are typically provided by L-acid [25], while medium-strong acid sites originate from B-acid [25]. Enhanced acidity levels facilitate the adsorption and activation of C=O and C-O [35–37], thereby promoting deep hydrogenation reactions. To characterize the type of acidic sites in the catalyst, pyridine adsorption FTIR (Py-FTIR) analysis was conducted (Fig. 2d), revealing that SiO₂ introduction elevates the content of L-acid and B-acid sites. L-acid sites are provided by Si⁺-O defects in SiO₂ [26] and low-valence Mo species [38], whereas B-acid sites are facilitated by Si-OH-metal interactions [26]. This can be attributed to the formation of rich SiO₂-Ni₃Mo₃N interface. This joint adsorption between SiO₂ and Ni₃Mo₃N for raw material and intermediate products ultimately promotes EtOH formation.

To further investigate the enhancement of catalytic activity by SiO₂, hydrogen temperature-programmed desorption (H₂-TPD) analysis was performed on the catalysts before and after SiO₂ loading (Fig. S9 in Supporting information). The H₂ adsorption amount of the different catalysts were quantitatively calculated and are presented in Table S4. The results indicate that the Ni₃Mo₃N catalyst exhibits a higher H₂ adsorption amount, likely due to its abundance of Ni⁰ species, which facilitate H₂ adsorption [39], as supported by XPS characterization (Fig. S5a and Table S1 in Supporting information). Although the Ni₃Mo₃N catalyst exhibits a significant amount of H₂ adsorption, SiO₂-Ni₃Mo₃N demonstrates even higher amount of H₂ adsorption within the high temperature range (300–600 ℃). Moreover, the strength of H₂ adsorption increases with rising temperatures [40]. This indicates that SiO₂-Ni₃Mo₃N possesses superior H₂ adsorption capabilities, facilitating deep hydrogenation reactions and promoting EtOH formation.

The optimization of the SiO₂-Ni₃Mo₃N catalyst's reaction conditions was conducted. Fig. S10a (Supporting information) illustrates the optimization of reaction temperature (T_reaction). The findings indicate that MA is the predominant by-product of this catalyst. Remarkably, an EtOH yield of 97.5% is achieved at a reaction temperature of 210 ℃. Even at a lower temperature of 180 ℃, an EtOH yield of 85% is attainable, enabling low-temperature EtOH production. Another key factor influencing catalytic activity is reaction pressure (P_reaction), which was also optimized (Fig. S10b in Supporting information). The results show that the highest EtOH yield of 98% is achieved at a pressure of 2 MPa, with further pressure increases having negligible impact on catalyst activity. Notably, even at a lower pressure of 1.5 MPa, an EtOH yield exceeding 95% is still obtained. The effect of the molar ratio of H₂ to DMO (H₂/DMO (mol)) on catalytic performance was also investigated (Fig. S10c in Supporting information). The findings revealed that reducing H₂/DMO (mol) from 200 to 50 had minimal impact on EtOH yield, with only a slight decrease observed. Screening for weight liquid-hour space velocity (WLHSV_DMO) of DMO (Fig. S10d in Supporting information) demonstrated that higher WLHSV_DMO conditions led to increased formation of by-product MA. This is attributed to shorter residence times of raw material and intermediates on the catalyst due to elevated WLHSV_DMO, resulting in insufficient hydrogenation of intermediates for EtOH production. These results indicate that the optimal reaction conditions are T_reaction = 210 ℃, P_reaction = 2 MPa, H₂/DMO (mol) = 50, and WLHSV_DMO = 0.1 h^-1. Life tests conducted under these optimal conditions (Fig. 10e in Supporting information) demonstrated that the catalyst maintained exceptional catalytic activity for 240 h. Compared to previous studies (Table S5 in Supporting information), this study achieved a high EtOH yield (95%−97.5%) at a low temperature of 210 ℃ and relatively low pressure (1.5–2 MPa). Additionally, the H₂/DMO (mol) ratio of 50 greatly facilitates the industrialization process of synthesizing EtOH from synthesis gas.

To investigate the reaction mechanism of DMO hydrogenation to EtOH on this catalyst, in-situ diffuse reflectance infrared Fourier-transform spectroscopy (in-situ DRIFTS) analysis was conducted at 210 ℃. The results are depicted in Fig. 3, where the peak at 1715 cm^-1 corresponds to the C=O stretching vibration of the carboxyl group in DMO [33]. Its intensity gradually decreases with increasing reaction time, indicating that C=O in DMO is adsorbed and activated by the catalyst (via L-acid sites) and subsequently hydrogenated to form the corresponding hydroxyl group, facilitating the formation of intermediate product MG. Notably, no distinct stretching vibration peak of -OH was observed within the range of 3500–3700 cm^-1. This may be due to the facile adsorption and activation of -OH from MG by the catalyst (via B-acid sites), leading to its conversion through hydrodehydration into subsequent products, as evidenced by experimental findings (with the main by-product being MA (Fig. 2a)). The in-situ DRIFTS analysis conducted at different temperatures also confirms that the -OH groups generated during initial hydrogenation are prone to undergo dehydration, resulting in the formation of MA. As depicted in Fig. S11 (Supporting information), the peak intensities corresponding to all observed peaks exhibit an increasing trend with rising temperature; however, the stretching vibration peak of -OH [41] start decreasing once the temperature reaches 210 ℃, indicating further occurrence of dehydration reactions. Additionally, a peak at approximately 2928 cm^-1 indicates CH₃O- formation (Fig. 3) [42]. With prolonged reaction time, the vibrational intensity of CH₃O- species gradually increases, suggesting that C-O in DMO is adsorbed by the catalyst (via B-acid sites) and cleaved to generate CH₃O-. Herein, C-O represents a carbon-oxygen single bond connecting oxygen from the methoxy group with carbon from the carboxyl group in DMO. This corresponds to a stretching vibration at 1157 cm^-1 for the C-O bond, which exhibits decreased intensity as reaction time progresses. Overall, the results indicate that on this catalyst, Scheme S1 (Supporting information) depicts the proposed reaction mechanism: Initially, DMO is adsorbated on the surface of the catalyst through C=O, and subsequent hydrogenation and pyrolysis reactions are carried out. With the pyrolysis of C-O, the primary product MG is formed, and the hydrolysis of MG is quickly carried out to form the main intermediate product MA; furthermore, MA undergoes further hydrogenation of its C=O moiety along with cleavage of its C-O bond, ultimately yielding the target product EtOH.

Figure 3

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Fig. 3. In-situ DRIFTS analysis of the SiO2-Ni3Mo3N catalyst.

Additionally, the mechanism was probed through DFT simulation calculations, constructing the catalyst model as depicted in Fig. S12 (Supporting information) based on experimental findings. The partial density of states (PDOS) diagram of the SiO₂-Ni₃Mo₃N catalyst (Fig. S13a in Supporting information) revealed overlapping Mo and N peaks, indicating Mo-N bond presence and confirming successful nitride formation [43], consistent with previous characterization results. Analysis of PDOS diagrams for various catalysts (Fig. 4a) demonstrated that SiO₂ introduction affected electron density distribution within Mo 3d orbitals, potentially due to electron transfer at the SiO₂-Ni₃Mo₃N interface. Differential charge density analysis clearly illustrated electron transfer between Ni₃Mo₃N and SiO₂ (Fig. 4b), with Ni₃Mo₃N transferring 1.42 eV to SiO₂ (Fig. 4c), aligning with previous XPS results that support SiO₂-Ni₃Mo₃N interface formation. This electron transfer also adjusts the d-band center of the catalyst. Further calculations indicated that introducing SiO₂ raised the d-band center by 0.09 eV for Mo 3d orbitals in Ni₃Mo₃N catalysts (Fig. 4a), bringing it closer to the Fermi level (0 eV). The submatter-adsorbent interaction is enhanced [44,45], thereby facilitating catalyst adsorption onto raw material or intermediate products and augmenting its hydrogenation ability towards EtOH. Additionally, a similar trend was observed for d-band centers in Ni 3d orbitals (Fig. S13b in Supporting information), further supporting that SiO₂ enhances the catalyst's adsorption capabilities. The adsorption energy of the catalyst on the raw material and intermediate products was compared, as presented in Table S6 (Supporting information). Upon SiO₂ introduction, the catalyst exhibited a higher negative adsorption free energy on the substrate (DMO, MG, MA), indicating enhanced adsorption capacity facilitated by SiO₂. Simultaneously, analysis of differential charge density after DMO adsorption onto the catalyst (Fig. S14 in Supporting information) revealed electron transfer between DMO and the catalyst, with the SiO₂-Ni₃Mo₃N catalyst demonstrating greater electron transfer with DMO. These findings indicate that SiO₂ presence facilitates electron transfer, serving as a mediator to enhance DMO adsorption. This is further corroborated by analyzing the differential charge density of corresponding intermediates adsorbed on the catalyst (Fig. S15 in Supporting information).

Figure 4

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Fig. 4. (a) The fractional state density of catalysts. (b) The differential charge density of the SiO2-Ni3Mo3N catalyst (purple and green areas denote electron gain and loss, respectively). (c) The optimized structure. The N, Ni, Mo, Si, and O atoms are represented by dark blue, light blue, cyan, yellow, and red colors, respectively.

To elucidate the DMO adsorption sites of the catalyst, adsorption energies of DMO at both Ni (Fig. S16 in Supporting information) and Mo (Fig. S14a) sites were calculated using the Ni₃Mo₃N catalyst, as summarized in Table S7 (Supporting information). Both adsorption energies are negative, indicating effective DMO adsorption and subsequent hydrogenation reaction at both sites. However, the greater adsorption energy of Mo towards DMO suggests that DMO primarily undergoes adsorption through Mo sites on the catalyst surface [44], consistent with previous findings. Additionally, to determine the activation sites for H adsorption on the catalyst, separate calculations of the adsorption energy of H₂ at both Ni and Mo sites were performed (Fig. S17 and Table S7 in Supporting information). The negative adsorption energies indicate that both Ni and Mo sites can adsorb H₂ for deep DMO hydrogenation. Nevertheless, Ni exhibits a stronger affinity for H₂ adsorption, implying that the catalyst's hydrogen adsorption and activation primarily rely on Ni sites, aligning with prior investigations [39].

DFT simulation reveals that incorporating SiO₂ facilitates electron transfer of SiO₂-Ni₃Mo₃N interface, modulates the d-band center of the catalyst, enhances adsorption capabilities towards raw materials and intermediates, and boosts catalytic activity. According to DFT simulation calculations, Mo sites primarily serve to adsorb raw materials and intermediates, while Ni predominantly facilitates H₂ adsorption and activation

This study focuses on preparing a catalyst with a SiO₂-Ni₃Mo₃N interface for DMO hydrogenation to produce EtOH, achieving an impressive EtOH yield of 97.5% even at a low temperature of 210 ℃. Notably, the EtOH yield remains above 95% under conditions of 210 ℃ and 1.5 MPa. The research demonstrates that SiO₂ is uniformly distributed on the Ni₃Mo₃N surface, facilitating electron transfer and forming an enriched SiO₂-Ni₃Mo₃N interface. Additionally, the introduction of SiO₂ enhances both L-acid sites (facilitating C=O adsorption and activation) and B-acid sites (promoting C-O adsorption and activation), thus promoting the adsorption of raw material and intermediate products. Concurrently, SiO₂ incorporation increases the H₂ adsorption capacity, significantly enhancing the catalyst's deep hydrogenation capability. The reaction mechanism was investigated through in-situ DRIFTS characterization, revealing that the catalyst first adsorbs and activates C=O in DMO and along with the cleavage of C-O to the generate intermediate product MA, which is further hydrogenated by C=O and C-O cleavage to produce the target product, EtOH. DFT simulation revealed that the presence of SiO₂ facilitated electron transfer at the SiO₂-Ni₃Mo₃N interface, adjusted the catalyst's metal d-band center, promoted electron transfer between the catalyst and substrate, and enhanced the catalyst's adsorption capacity for raw material and intermediates, thereby augmenting its hydrogenation ability towards EtOH. This study successfully achieves low-temperature efficient EtOH synthesis, advances industrial processes from syngas to EtOH production, and provides new insights into constructing highly active catalytic interfaces for DMO hydrogenation.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Jiang Gong: Writing – original draft, Validation, Methodology, Investigation, Formal analysis, Data curation. Fengling Zheng: Writing – review & editing, Visualization, Validation. Hanqing Zhang: Writing – review & editing, Validation. Weihan Shu: Writing – review & editing. Hao Wang: Writing – review & editing. Ni Zhang: Writing – review & editing. Pengbing Huang: Writing – review & editing. Chuancai Zhang: Writing – review & editing, Supervision, Project administration, Funding acquisition, Conceptualization. Bin Dai: Supervision, Funding acquisition, Conceptualization.

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21962015) and the Bingtuan Graduate Innovation Project 2024 (No. BTYJXM-2024-K12).

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111122.